With the advent of optical waveguides for use in the telecommunications industry, much emphasis has recently been placed on vapor deposition as a materials forming technique. In constructing preforms from which optical fibers may be drawn, vapors of materials such as SiCl.sub.4, SeCl.sub.4, and PoCl.sub.3 must be precisely blended and delivered at controlled mass flow rates to a preform construction site where they are reacted and deposited on or within a support. This can be done by passing carrier gases such as H.sub.2, He, N.sub.2, O.sub.2, or Ar through supplies of the materials in liquid form to the deposition site with the vapors entrained with the carrier gas. In performing this operation a group of vaporizors is normally used of the type known as bubblers.
A bubbler has a chamber in which a gas intake conduit terminates with an outlet orifice located below the surface of liquid contained therein. An outlet conduit communicates between the space above the surface of the liquid and the vapor deposition site. Exemplary of deposition systems employing bubblers is that illustrated in U.S. Pat. No. 3,826,560.
Since vapors of the liquid housed within a bubbler are intermittently or continuously being withdrawn during bubbler operation, the level of liquid descends unless replenished from an auxiliary source. In some applications such decreases in the level of liquid within the bubbler would have little effect. In other applications, however, such as in vapor deposition procedures employed in constructing optical fiber preforms, significant variations in the level of liquid have a pronouncely adverse effect. This is attributible to the fact that the rate of vaporization is not solely dependent upon the surface area of liquid within the bubbler which area can, of course, be maintained constant as by the use of cylindrically shaped vessels. The vaporization rate here however is also dependent upon several other factors including the flow characteristics of carrier gas bubbled through the liquid. For example, the size of the bubbles as they rise through the liquid has an effect on the rate of vaporization. The rate of flow of the carrier gas introduced into the bubble itself has another effect on the rate of vaporization as also does the rise time of the bubbles which, of course, depends on the depth at which they are introduced. The rate of heat transfer into the bubbler will also be effected by changes in the level of liquid. While theoretically possible to program a heater controller to account for these variables as changes in the level of liquid are continuously monitored, that approach is complex and fails to eliminate the need for some degree of level control to prevent complete depletion or flooding.
Accordingly, it is to the task of maintaining a substantial constant level of liquid within a bubbler while vapors from the liquid are being continuously or intermittantly withdrawn to which the present invention is generally directed. More particularly, the invention is directed to level control systems and methods for use in vaporizing highly corrosive liquids such as halogens which are used in constructing optical fiber preforms and which may easily become contaminated if brought into direct contact with materials of the type used in conventional level controllers such as floats and the like.